1. Field of the Invention
This invention relates to seismometers for detecting transient seismic signals in the earth and particularly to a sensor for detecting orthogonal components of such signals on land or in shallow water environments.
2. Brief Description of the Related Art
Seismic exploration involves the surveying of subsurface layers using reflected acoustic impulses artificially introduced in the subsurface. To conduct such a survey, a array of seismic cables may be located along the surface of the earth, interconnecting strings of geophones to a centrally-located recording station. Traditionally, each geophone consisted of a single sensing unit oriented perpendicular to the surface of the earth so as to detect upward propagating waves, and thus was only able to record one component of the seismic signal, e.g. the vertical component. However, horizontal components of a seismic signal have become of extreme interest to the geophysicist. To detect horizontal and vertical components of a seismic signal, three geophones are often employed, each located in a single plane with their axes of sensitivity oriented perpendicular to each other. In this configuration, the three sensors detect the two horizontal components and the vertical component which are computationally combined to determine subsurface propagation characteristics.
Prior three-dimensional (3-D) geophone units employed three individual geophones, each disposed in the same plane with their axes of sensitivity oriented orthogonal to each other. Oyo Corporation's Model SC-3D and Mark Products' Model L-400 have a circular housing with the three geophones located equidistant from each other in the same plane. LRS (formerly Litton Resources Systems, Inc.), a division of Western Atlas International, Inc., employs a rectangular housing with the three geophones disposed along the longitudinal axis of the housing. These 3-D geophones are much larger and heavier in comparison to single geophone units. Since each unit is larger, each seismic cable must be shorter to enable the field hand to carry the geophones to the surface location. Thus more man-power is expended to place the same number of 3-D geophones than to place the same number of single geophone units. There has been a long felt yet unresolved need for a more compact and lighter 3-D geophone assembly in order to provide the same subsurface coverage as in single geophone applications.
Geophones in general have a tendency to flood when exposed to shallow bodies of water. The geophone units mentioned above also suffer this problem. One particular source of flooding is the electrical cable entrance into the housing. There has been a long felt and unsolved need for a geophone housing rugged enough to withstand prolonged field use and yet retain its watertight integrity.
In light of these prior disadvantages, it is an object of this invention to provide a 3-D geophone of reduced size for ease in handling, thus enabling more geophones per unit length of cable. It is another object of this invention to provide a 3-D geophone which is water-proof/resistant when exposed to shallow water environments.